Web Mapping Illustrated

Using Projections with Other Applications

MapServer is just one application of many that use PROJ.4 to help project coordinates. Two other utilities discussed elsewhere in this book can use the power of PROJ.4 to project data/features into new projections. This is often accomplished using a few command-line options. The gdalwarp and ogr2ogr command-line tools (part of the GDAL/OGR package hosted at http://gdal.org) allow you to convert raster and vector data, respectively, between formats. You can also specify a target spatial reference system using the -t_srs option. For example, to convert an image that is referenced to latitude/longitude coordinates, into a Mercator projection, the following command might be used:

    > gdalwarp -t_srs "+proj=merc +ellps=WGS84" in.tif out.tif

There is also an option, -s_srs, that allows you to specify the source SRS. This is essential if the data you have isn't already encoded with projection information.

Here is another example; it uses the ogr2ogr vector conversion utility to convert a shapefile from one SRS to another. The source SRS is also supplied because there was no projection information (or .prj file) supplied with the data.

    > ogr2ogr -s_srs "+proj=latlong" -t_srs "+proj=merc +ellps=WGS84" city_merc.shp city_in.shp